Agonistic antibodies stimulate the kinase encoded by the neu protooncogene in living cells but the oncogenic mutant is constitutively active.

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Abstract
The neu protooncogene (also called c-erbB2 and HER-2) undergoes oncogenic activation through a single mutation. The product of the protooncogene, p185neu, probably functions as a receptor for a peptide growth factor. To circumvent the absence of a well-characterized ligand, I generated ligand-mimicking monoclonal antibodies directed to the presumed receptor. These antibodies stimulated tyrosine phosphorylation of p185neu in living cells and also accelerated the rate of endocytosis and degradation of p185neu. A monovalent Fab fragment of such an antibody was ineffective, suggesting a role for receptor dimerization in signal transduction. Unlike the product of the protooncogene, the transforming mutant was not affected by the ligand-like antibodies. However, it undergoes constitutively high phosphorylation on tyrosine residues in living cells, and its turnover rate is remarkably rapid. Nevertheless, the pattern of phosphorylation of the mutant protein is similar to the one exhibited by an antibody-stimulated p185neu, suggesting that the mutation mimics activation by the antibody. These results suggest that the kinase of p185neu is under allosteric control that may involve ligand-induced dimerization of receptors. This mechanism is deregulated in the oncogenic mutant, which is functionally equivalent to ligand-stimulated receptor.